Organic electroluminescent devices are attracting public attention as promising display devices for capable of emitting light of high luminance with low voltage. An important characteristic of organic electroluminescent devices is consumed electric power. Consumed electric power is equal to the product of the voltage and the electric current, and the lower the value of voltage that is necessary to obtain desired brightness and the smaller the value of electric current, the lower is the consumed electric power of the device.
As one attempt to lower the value of electric current that flows to a device, a light-emitting device utilizing luminescence from ortho-metalated iridium complex (Ir(ppy)3: Tris-Ortho-Metalated Complex of Iridium(III) with 2-Phenylpyridine) is reported (see JP-A-2001-247859). The phosphorescent devices disclosed therein are greatly improved in external quantum efficiency as compared with singlet luminescent devices in the related art, and have succeeded in making the value of electric current smaller. For the purpose of more enhancing efficiency and more improving durability of the phosphorescence device, devices containing a platinum complex having a tetradentate ligand have been reported (see WO 2004/108857).
On the other hand, for the purpose of improving luminance and efficiency, light-emitting devices using an indole derivative as a host material have also been reported (see, for example, JP-A-2002-305084 and JP-A-2003-277744). These light-emitting devices have succeeded in improving efficiency thereof, but more improvement has been desired in view of durability, reduction in driving voltage, and more enhancement of efficiency.